But on Monday, Brown announced a climate goal so ambitious that many experts don’t know how to reach it.

As diplomats descended on San Francisco for a global climate conference, Brown signed an executive order calling for the state to slash its overall emissions to zero by 2045 and then go negative. Starting in 2046, California would pull more carbon dioxide and other greenhouse gases from the atmosphere, year-by-year, than it put in.

The governor also signed a bill, SB100, that calls for 100 percent of the state’s electricity to come from carbon-free sources by 2045.

Brown’s signature on the clean-energy bill was widely expected, but the executive order was not. California had previously set a target of cutting the state’s greenhouse gas emissions 80 percent by 2050 — already considered a difficult task.

Brown has made global warming a signature issue of his second stint in the governor’s office, although he has clashed with activists who want him to phase out oil production in the state. He said Monday that California’s new goals show its commitment to the landmark 2015 international Paris Agreement on climate change, despite the Trump administration’s opposition.

“It’s not going to be easy, and it won’t be immediate, but it must be done,” Brown said, shortly before signing the bill and the executive order. “California is committed to doing whatever is necessary to meet the existential threat of climate change and, yes, it is an existential threat.”

Although California in the past decade has rapidly increased its use of renewable power, experts warn that reaching 100 percent clean energy at a cost consumers can afford will require better and cheaper ways to store large amounts of electricity.

But removing carbon from the atmosphere en masse is a far more daunting task. While several technologies have been explored — incorporating carbon dioxide into concrete, using ocean algae blooms to absorb the gas — none has been deployed at scale.

The California Air Resources Board, which runs most of the state’s global warming programs, calls for sequestering 5 million metric tons of carbon per year through protecting, managing and expanding forests. Compared with the size of California’s current emissions, that isn’t much. In 2016, the latest year for which data are available, the state’s economy emitted 429.4 million metric tons of greenhouse gases.

And wildfires can pump so much carbon dioxide into the atmosphere that a single large blaze can wipe out the state’s success in trimming emissions for that entire year. Fires have increased in size and number in recent years, and many experts fear climate change could continue that trend.

“Ultimately, we have to use our land in a very thoughtful way in order to go net negative,” said Peter Miller, western energy project director for the Natural Resources Defense Council environmental group. “It’s very challenging.”

One potential approach, he said, would be to grow crops specifically for use in biomass-fueled power plants, then capturing the plants’ emissions and pumping them into geologic formations underground. Although the technology exists to trap and store emissions, an idea called carbon capture and storage, it is considered far too expensive for widespread deployment.

Miller said, however, that setting a goal of going carbon negative could spur a serious discussion of how to get there.

“Given that we’ve got positive emissions now and for a number of years, at some point we’re going to have to figure out how to do that,” he said. “There’s a challenge, and we’re going to have to rise to it.”

David Baker covers energy, clean tech, electric vehicles and self-driving cars for the San Francisco Chronicle. He joined the paper in 2000 after spending five years in Southern California reporting for the Los Angeles Times and the Daily News of Los Angeles. He has reported from wind farms, geothermal fields, solar power plants, oil fields and an offshore drilling rig in the Gulf of Mexico. He also visited Baghdad and Basra in 2003 to write about Iraq's reconstruction. He graduated from Amherst College and the Columbia University Graduate School of Journalism. He lives in San Francisco with his wife.